Coxiella burnetti differs from other rickettsiae in that it undergoes a change, phase I yields phase II, during cultivation in chick embryos and may undergo the reverse, phase II yields phase I, in animals. The phase shift is characterized by loss or gain of phase I antigen and by changes in infectivity in cell culture systems, mice and guinea pigs, and by changes in pathogenicity in guinea pigs. The general purpose of this project is to study the mechanism of phase change and to examine the influence of phase on the behavior of C. burnetti in a variety of biological and physical systems.